1. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge
end systems, access networks, links
1.3 network core
packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 end-to-end throughput analysis
Introduction

2. What’s the Internet: a service view
Communication infrastructure
enables distributed applications
Web, VoIP, , games, e-commerce, social nets, …
Communication services
provides programming interface to apps
hooks that allow sending and receiving app programs to “connect” to Internet
provides service options, analogous to postal service
mobile network
global ISP
regional ISP
home
network
institutional
Introduction

3. What’s the Internet: “nuts and bolts” view
smartphone PC
server
wireless
laptop
hosts or end systems:
billions of connected computing devices
running network apps
mobile network
global ISP
regional ISP
home
network
institutional
communication links:
fiber, copper, radio, satellite
transmission rate: bandwidth
wired
links
wireless
routers & switches:
forward packets (chunks of data)
router
Introduction 3

4. What’s a protocol? protocols define
TCP connection
request
protocols define
format, order of messages sent and received among network entities,
and actions taken on message transmission, receipt
TCP connection
response
<file>
Get
Introduction

5. What’s the Internet: “nuts and bolts” view
mobile network
global ISP
regional ISP
home
network
institutional
Internet: “network of networks”
Interconnected ISPs
Internet standards
IETF: Internet Engineering Task Force
RFC: Request for comments
IEEE for links/hardware
Ethernet, ,
protocols
control sending, receiving of messages
e.g., TCP, IP, HTTP, Skype,
Introduction

6. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge
end systems, access networks, links
1.3 network core
packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 end-to-end throughput analysis
Introduction 6

7. A closer look at network structure:
network edge:
hosts: clients and servers
servers often in data centers
mobile network
global ISP
regional ISP
home
network
institutional
access networks, physical media:
wired, wireless communication links
network core:
interconnected routers
network of networks
Introduction

8. Access networks Q: How to connect end systems to edge router?
residential access nets
enterprise/institutional access networks
school, company,
mobile/wireless access networks
Introduction

9. to/from headend or central office
Residential access networks
wireless
devices
to/from headend or central office
often combined
in single box
wireless access
point (54 Mbps)
router, firewall, NAT
cable or DSL modem
wired Ethernet (1 Gbps)
Introduction

10. Enterprise access networks (Ethernet)
institutional link to
ISP (Internet)
institutional router
Ethernet
switch
institutional mail,
web servers
typically used in companies, universities, etc.
10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates
today, end systems typically connect into Ethernet switch
Introduction

11. Mobile/wireless access networks
shared wireless access network connects end system to router
via base station aka “access point”
wide-area wireless access
provided by telco (cellular) operator, 10’s km
between 1 and 10 Mbps
3G, 4G: LTE
wireless LANs:
within building (100 ft.)
802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate
to Internet
to Internet
Introduction

12. Host: sends packets of data
host sending function:
takes application message
breaks into smaller chunks, known as packets, of length L bits
transmits packet into access network at transmission rate R
link transmission rate, aka link capacity, aka link bandwidth
two packets,
L bits each 2 1
R: link transmission rate
host
L (bits)
R (bits/sec)
packet
transmission
delay
time needed to
transmit L-bit
packet into link = =
Introduction

13. Physical media bit: propagates between transmitter/receiver pairs
physical link: what lies between transmitter & receiver
guided media:
signals propagate in solid media: copper (twisted pair), fiber, coax
unguided media:
signals propagate freely, e.g., wireless
Twisted pair cable
Coaxial cable
fiber optic cable
Introduction

14. Unguided media radio link types:
terrestrial microwave
e.g. up to 45 Mbps channels
LAN (e.g., WiFi)
54 Mbps
wide-area (e.g., cellular)
4G cellular: ~ 10 Mbps
satellite
Kbps to 45Mbps channel (or multiple smaller channels)
270 msec end-end delay
signal carried in electromagnetic spectrum
no physical “wire”
bidirectional
propagation environment effects:
reflection
obstruction by objects
interference
Introduction

15. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge
end systems, access networks, links
1.3 network core
packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 end-to-end throughput analysis
Introduction 15

16. The network core The fundamental question:
how is data transferred through net?
circuit-switching: dedicated circuit per call: telephone nets
packet-switching: data sent thru net in discrete “chunks”
Introduction

17. Network core: packet-switching
100 Mb/s
Ethernet C A
1.5 Mb/s B
Packet-switching:
Source breaks application-layer message into packets
Forward from one router to the next until reaching destination
Introduction

18. Network core: packet-switching
100 Mb/s
Ethernet C A
1.5 Mb/s B
Packet-switching:
no dedication/reservation: all streams share resources
no setup is required
resources used as needed
each packet uses full link bandwidth
aggregate resource demand can exceed capacity
no guarantee
Introduction

19. Packet Switching: statistical multiplexing
R = 100 Mb/s
statistical multiplexing D
R = 1.5 Mb/s B E
queue of packets
waiting for output link
Sequence of A & B packets does not have fixed pattern, shared on demand  statistical multiplexing
Introduction

20. Packet-switching: store-and-forward
L bits
per packet 3 2 1
source
destination
R bps
R bps
takes L/R sec to transmit (push out) L-bit packet into link at R bps
store and forward: entire packet must arrive at router before it can be transmitted on next link
one-hop numerical example:
L = 7.5 Mbits
R = 1.5 Mbps
one-hop transmission delay = 5 sec
end-end delay = 2L/R (assuming zero propagation delay)
more on delay shortly …
Introduction

21. Packet Switching: queueing delay, loss
R = 100 Mb/s D
R = 1.5 Mb/s B E
queue of packets
waiting for output link
queuing and loss:
if arrival rate (in bits) to link exceeds transmission rate of link for a period of time, then there will be consequences:
delay: packets will queue, wait to be transmitted on link
loss: packets can be dropped if memory (buffer) fills up
Introduction

22. Two key network-core functions
routing: determines source- destination route taken by packets
routing algorithms
forwarding: move packets from router’s input to appropriate router output
routing algorithm
local forwarding table
header value
output link
0100
0101
0111
1001 3 2 1 1 2 3
0111
destination address in arriving
packet’s header
Introduction

23. Alternative core: circuit switching
reserved resources:
resources divided into ‘pieces’
end-end resources are reserved for each “call”
in diagram, each link has four circuits
call gets 2nd circuit in top link and 1st circuit in right link.
no sharing:
dedicated resources:
guarantees:
circuit-like performance
Introduction

24. Alternative core: circuit switching
wasted if not used:
circuit segment idle if not used by call (no sharing)
overhead:
call setup is required
admission control
same path for all chunks
commonly used in traditional telephone networks
Introduction

25. Circuit switching: FDM versus TDM
Multiplexing: Combining multiple signals such that individual signals can be retrieved at the destination. 
Signals occupy the same channel, so the channel must be shared
Two ways of dividing bandwidth into “pieces”
frequency division
time division
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction

26. Circuit switching: FDM versus TDM
4 users
Example:
Freq. Division Multiplexing (FDM)
frequency
time
Time Division Multiplexing (TDM)
frequency
time
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction

27. Circuit switching: FDM versus TDM
Frequency Division: Divides the channel into multiple, but smaller frequency ranges.
(+) Supports more user at one time
(+) Low latency, no time turn, preferred for real-time applications, limited
(-) But limited in terms of frequency availability
Time Division: Allocates certain time periods to each channel in an alternating manner.
(+) Much more flexible, can support many users but at different times
(-) Slower, have to wait the turn
What is the best option?
Two simple multiple access control techniques.
Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling.
As we will see, used in AMPS, GSM, IS-54/136
Introduction

28. Packet switching versus circuit switching
2 circuits (use TDM)
A reserves 1 circuit
B reserves 1 circuit
2 Mb/s B
Utilization = 50% only = 1 Mb/s
B: has no
packets to send A B
2 Mb/s
Packet switching
statistical multiplex.
B uses full link since
A is not using it
Utilization = 100% = 2 Mb/s
Introduction

29. Numerical example Solution:
How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network?
The link’s transmission rate = 0.64 Mbps
Each link uses TDM with 10 slots/sec
0.5 sec to establish end-to-end circuit
Solution:
Bandwidth of circuit (in kbps)= .64x1000/10 = 64 kbps
Time to send: 640 kbits/64 kbps + 0.5s = 10.5s
Introduction

30. Packet switching versus circuit switching
resources
sharing
no sharing
congestion
may lead to
needs congestion control
admission control
overhead
less overhead
no connection setup
more overhead
need to reserve resources first
guarantee
best-effort model
no guarantees
provides guarantees
good for multimedia
Introduction

31. Packet switching versus circuit switching
packet switching allows more users to use network!
3 Mb/s link
each user:
1 Mb/s when “active”
active 1/3 of time
circuit-switching:
3 users
packet switching:
With N=4 users, what are the chances that a user won’t get 1 Mb/s?
I.e., what is the prob. that more than 3 (strictly) users are active?
With N=5 users, what are the chances that a user won’t get 1 Mb/s?
…….
….. N
users
3 Mbps link
Introduction

32. Internet structure: network of networks
Question: given millions of access ISPs, how to connect them together?
access
net …
Introduction

33. Internet structure: network of networks
Option: connect each access ISP to every other access ISP? … …
access
net
access
net …
access
net
access
net
access
net
access
net
access
net
connecting each access ISP to each other directly doesn’t scale: O(N2) connections. … …
access
net
access
net
access
net
access
net
access
net
access
net …
access
net
access
net …
access
net
Introduction

34. Internet structure: network of networks
Option: connect each access ISP to one global transit ISP?
Customer and provider ISPs have economic agreement.
access
net …
global ISP
Introduction

35. Internet structure: network of networks
But if one global ISP is viable business, there will be competitors …. … …
access
net
access
net
access
net
access
net
access
net
access
net
access
net
ISP A … …
ISP B
access
net
access
net
ISP C
access
net
access
net
access
net
access
net …
access
net
access
net …
access
net
Introduction

36. Internet structure: network of networks
But if one global ISP is viable business, there will be competitors …. which must be interconnected
Internet exchange point … …
access
net
access
net
access
net
access
net
access
net
IXP
access
net
access
net
ISP A … …
IXP
ISP B
access
net
access
net
ISP C
access
net
peering link
access
net
access
net
access
net …
access
net
access
net …
access
net
Introduction

37. Internet structure: network of networks
… and regional networks may arise to connect access nets to ISPs … …
access
net
access
net
access
net
access
net
access
net
IXP
access
net
access
net
ISP A … …
IXP
ISP B
access
net
access
net
ISP C
access
net
access
net
access
net
regional net
access
net …
access
net
access
net …
access
net
Introduction

38. Internet structure: network of networks
… and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users … …
access
net
access
net
access
net
access
net
access
net
IXP
access
net
access
net
ISP A … …
Content provider network
IXP
ISP B
access
net
access
net
ISP C
access
net
access
net
access
net
regional net
access
net …
access
net
access
net …
access
net
Introduction

39. Internet structure: network of networks
Tier 1 ISP
Tier 1 ISP
Google
IXP
IXP
IXP
Regional ISP
Regional ISP
access
ISP
access
ISP
access
ISP
access
ISP
access
ISP
access
ISP
access
ISP
access
ISP
at center: small # of well-connected large networks
“tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage
content provider network (e.g., Google): private network that connects it data centers to Internet, often bypassing tier-1, regional ISPs
Introduction

40. Tier-1 ISP: e.g., Sprint
Introduction

41. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge
end systems, access networks, links
1.3 network core
packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 end-to-end throughput analysis
Introduction 41

42. Four sources of packet delay
propagation
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop A B
transmission
dproc: nodal processing
check bit errors
determine output link
typically < msec
dqueue: queueing delay
time waiting at output link for transmission
depends on congestion level of router
Introduction

43. Four sources of packet delay
transmission A
propagation B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dtrans: transmission delay:
L: packet length (bits)
R: link bandwidth (bps)
dtrans = L/R
dprop: propagation delay:
d: length of physical link
s: propagation speed (~2x108 m/sec)
dprop = d/s
dtrans and dprop
very different
* Check out the online interactive exercises for more examples:
* Check out the Java applet for an interactive animation on trans vs. prop delay
Introduction 43

44. “Real” Internet delays and routes
what do “real” Internet delay & loss look like?
traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i:
sends three packets that will reach router i on path towards destination
router i will return packets to sender
sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Introduction

45. “Real” Internet delays, routes
traceroute: gaia.cs.umass.edu to
3 delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw ( ) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu ( ) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu ( ) 6 ms 5 ms 5 ms
4 jn1-at wor.vbns.net ( ) 16 ms 11 ms 13 ms
5 jn1-so wae.vbns.net ( ) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu ( ) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu ( ) 22 ms 22 ms 22 ms
( ) 104 ms 109 ms 106 ms
9 de2-1.de1.de.geant.net ( ) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net ( ) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net ( ) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr ( ) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr ( ) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr ( ) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net ( ) 135 ms 128 ms 133 ms
( ) 126 ms 128 ms 126 ms
17 * * *
18 * * *
19 fantasia.eurecom.fr ( ) 132 ms 128 ms 136 ms
trans-oceanic
link
* means no response (probe lost, router not replying)
Introduction

46. Exercise 1 Answer: Question:
Packet length = L bits
trans. rate R = 1 Mbps
Host A
Host B
distance = 1 km, speed = 2x108m/s
Question:
Which bit is being transmitted at the time the first bit arrives at Host B
Answer:
First bit arrives after
1/R + d/s = 1/ /(2x108) = x10-6 = 6x10-6 = 6 µsec
After 6 µsec
6 bits are already transmitted; so 7th bit is being transmitted
Introduction

47. Exercise 2 Answer: Question: Transmission vs. propagation
L=100Bytes
trans. rate R = ?
Host A
Host B
distance = 2 km, speed = 2x108m/s
Question:
At what rate (bandwidth) R would the propagation delay equal the transmission delay of packet?
Answer:
Propagation delay = 2x103 (m)/2x108 (m/s) = 10-5 sec
Transmission delay = 100x8 (bits)/R
Prop. delay = trans. delay => R=105x100x8 = 80 Mbps
Introduction

48. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge
end systems, access networks, links
1.3 network core
packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 end-to-end throughput analysis
Introduction 50

49. is there any hope of organizing structure of network?
Protocol “layers”
Networks are complex,
with many “pieces”:
hosts
routers
links of various media
applications
protocols
hardware, software
Question:
is there any hope of organizing structure of network?
Introduction

50. Internet protocol stack
application: supporting network applications
FTP, SMTP, HTTP
transport: process-process data transfer
TCP, UDP
network: routing of datagrams from source to destination
IP, routing protocols
link: data transfer between neighboring network elements
Ethernet, (WiFi), PPP
physical: bits “on the wire”
application
transport
network
link
physical
Introduction

51. Encapsulation source destination application transport network link
message M
application
transport
network
link
physical
segment Ht M Ht
datagram Ht Hn M Hn
frame Ht Hn Hl M
link
physical
switch
destination
network
link
physical Ht Hn Hl M Ht Hn Hl M
application
transport
network
link
physical Ht Hn M
router
Introduction

52. ISO/OSI Model application application transport presentation network
link
physical
presentation
session
transport
network
data link
physical
5-layer Internet
Protocol Stack
7-layer ISO/OSI model
(OSI: open system interconnections)
Introduction 54

53. Chapter 1: roadmap 1.1 what is the Internet? 1.2 network edge
end systems, access networks, links
1.3 network core
packet switching, circuit switching, network structure
1.4 delay, loss, throughput in networks
1.5 protocol layers, service models
1.6 end-to-end throughput analysis
Introduction 55

54. Nodal delay: four delay components
dproc = processing delay
typically a few microsecs or less
dqueue = queuing delay
depends on congestion
dtrans = transmission delay
= L/R, significant for low-speed links
dprop = propagation delay
a few microsecs to hundreds of msecs
Introduction 56

55. How does packet loss occur?
queue (aka buffer) preceding link has finite capacity
packets may arrive at rate exceeding output link capacity
packets are queued in router buffers
packet arriving to full queue are dropped (aka lost)
lost packet may be retransmitted by previous node, by source end system, or not at all
buffer
(waiting area)
packet being transmitted A B
packet arriving to
full buffer is lost
* Check out the Java applet for an interactive animation on queuing and loss
Introduction

56. Queueing delay (more insights)
Packet arrival rate
= a packets/sec
Link bandwidth
= R bits/sec
Packet length
= L bits
Every second: aL bits arrive to queue
Every second: R bits leave the router
Question: what happens if aL > R ?
Answer: queue will fill up, and packets will get dropped!!
aL/R is called traffic intensity
Introduction

57. Queueing delay (more insights)
Link bandwidth
= R bits/sec
queue
1 packet arrives
every L/R seconds
Packet length L bits
Arrival rate: a = 1/(L/R) = R/L (packet/second)
Traffic intensity = aL/R = (R/L) (L/R) = 1
Average queueing delay = 0
(queue is initially empty)
Introduction

58. Queueing delay (more insights)
Packet arrival rate
= a packets/sec
Link bandwidth
= R bits/sec
Packet length
= L bits
La/R  1
La/R ~ 0
La/R ~ 0: avg. queueing delay small
La/R -> 1: delays become large
La/R > 1: more “work” than can be serviced, average delay infinite!
(this is when a is random!)
Introduction

59. Store-and-forward (revisited)L R R R
Entire packet must arrive at router before it can be transmitted on next link: store and forward
Takes L/R seconds to transmit (push out) packet of L bits on to link of R bps
delay = 3L/R (assuming zero propagation delay)
more on this next…
Introduction

60. Store-and-forward (more insights)
distance = d meters; speed of propagation = s m/sec
transmission rate of link = R bits/s R L
d/2 R L d
delay (one packet only)
= L/R + d/s
delay (one packet only)
= L/R + ½d/s + L/R + ½d/s
= 2L/R + d/s
Example:
d/s = 0.5 sec
L = 10 Mbits
R = 1 Mbps
delay = 10.5 sec
Example:
d/s = 0.5 sec
L = 10 Mbits
R = 1 Mbps
delay = 20.5 sec
Introduction

61. Store-and-forward (more insights)
distance = d meters; speed of propagation = s m/sec
transmission rate of link = R1 and R2 bits/s
Consider sending two packets A and B back to back
d/2
d/2 R1 R2 L
Case 2: Assume R1 > R2
Case 1: Assume R1 < R2
Q: is there a queuing delay? how much is this delay?
Answer (queue is empty initially):
Time for last bit of 2nd pkt to arrive at router: t1= L/R1 + L/R1 + d/(2s)
Time for last bit of 1st pkt to leave router: t2= L/R1 + d/(2s) + L/R2
Queueing delay = t2 – t1 = L/R2 – L/R1 if positive, otherwise 0. Hence:
when R1 < R2, queueing delay = t2 – t1 = 0
when R1 > R2, queueing delay = t2 – t1 = L/R2 – L/R1
Introduction

62. Throughput
throughput: rate (bits/time unit) at which bits transferred between sender/receiver
instantaneous: rate at given point in time
average: rate over longer period of time
server, with
file of F bits
to send to client
link capacity
Rs bits/sec
server sends bits
(fluid) into pipe
pipe that can carry
fluid at rate
Rs bits/sec)
Rc bits/sec)
link capacity
Rc bits/sec
Introduction

63. Throughput (more) Rs < Rc What is average end-end throughput?
Rc bits/sec
Rs bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
Bottleneck link on end-end path is what constrains end-end throughput
bottleneck link
Introduction

64. Throughput: Internet scenario
per-connection end-end throughput: min(Rc,Rs,R/10)
in practice: Rc or Rs is often bottleneck Rs Rs Rs R Rc Rc Rc
10 connections (fairly) share backbone bottleneck link R bits/sec
* Check out the online interactive exercises for more examples:
Introduction

65. Throughput analysis Question 1: how long it takes to send the file?
Host A
Host B R R R
Suppose: Host A has huge file of size F bits to send to Host B
File is split into N packets, each of length L bits (i.e., N=F/L)
Ignore propagation delay for now
Question 1: how long it takes to send the file?
A: (N+2)L/R = (F+2L)/R
Question 2: what is the average end-to-end throughput achieved when sending the file?
A: NL/[(N+2)L/R]=NR/(N+2) = FR/(F+2L) = R/(1+2L/F)
Introduction

66. Introduction: summary
Covered a “ton” of material!
Internet overview
Network protocol
Network edge, core, access network
Packet-switching versus circuit-switching
Internet/ISP structure
layering and service models
performance: delay and throughput analysis
Introduction